1. Field of the Invention
The present invention relates to curable elastomer compositions and reformable thermoplastic compositions which are reshapable subsequent to cure or solidification. In particular, the present invention relates to compositions and articles of manufacture which are prepared from curable elastomeric or deformable thermoplastic matrices having distributed therein polymeric powders which have a melting point below the degradation of the matrix and which are present in shape-holding amounts to permit reshaping subsequent to elastomer cure.
2. Brief Description of Related Technology
Thermoplastics are generally described as polymeric materials consisting of linear long chain molecules which exhibit crystalline and amorphous phases and which exhibit two transitions as a function of temperature (1) a second order transition where softening occurs, called the glass transition temperature (Tg), and (2) a first order transition, called Tm, where melting occurs. Because of the melting behavior, these materials can be reprocessed at elevated temperatures, to any shape desirable. Below the Tg, these materials are strong, stiff materials; above Tg but below Tm, they are strong materials, but less stiff and more extensible. See Principles of Polymerization, George Odion, Third Edition, John Wiley & Sons, Inc. (1988).
Elastomer or rubbery materials are linear long chain molecules which have very low Tg's and Tm's (if any, usually below room temperature), have low strength and very high extensibility. Crosslinking the elastomer improves strength and stiffness and decreases extensibility.
Thermoplastic elastomers are polymeric materials which possess properties of both thermoplastic and rubbery materials. Since these are mostly linear chains, they can be reprocessed at elevated temperatures, much like thermoplastics.
When long chain molecules are linked to each other at points other than their ends, the polymers are said to be crosslinked. When the number of crosslinks is sufficiently high, a three dimensional or space network polymer is produced in which substantially all the polymer chains in a sample have been linked to form one great molecule. Light crosslinking sometimes is used to impart good recovery (elastic) properties to polymers to be used as rubbers. A high degree of crosslinking is used to impart high strength, high rigidity and dimensioned stability at elevated temperatures. These highly crosslinked polymers are also called thermoset polymers.
Interpenetrating polymer networks (IPNs) are generally described as materials containing at least two polymers, each in a network or crosslinked form, with the two polymers having polymerized independently in the presence of each other to form two networks which are interwoven (interpenetrated) with each other. In other words, two or more polymerization reactions occur, resulting in separate but interwoven polymers. In some cases, slight crosslinking between the two polymers can also occur. If one or more of the two polymers are elastomeric initially, then the new polymer can be characterized as an elastomeric IPN.
A semi or pseudo-IPN is considered to be a material having only one crosslinked or network structure which is within a non-crosslinked polymer matrix phase, or vice versa. It is conceivable that certain solvents can extract this non-crosslinked phase of the semi-IPN, whereas this is generally not possible for an IPN. Therefore, a single crosslinked network of the semi-IPN allows these materials to retain thermoplastic character. They can be reformed with heat from one shape to another, providing the crosslinked network of the semi-IPN has a degree of elastomeric character, such as silicone segments within the crosslinked structure. Rigid crosslinked semi-IPNs derived from epoxy, bismaleimides or cyanates do not have this reforming ability.
Various patents disclose the case of thermoplastic components in combination with thermoset components. Much of the prior published subject matter modifies the properties of the thermoset by dynamically crosslinking the thermoset within the thermoplastic, i.e., chemically incorporating the thermoplastic into the crosslinked reaction product, rather than maintaining the thermoplastic in a distinct meltable phase within a cured pseudo or fully interpenetrating network structure, as in the present invention.
For example, U.S. Pat. No. 6,013,715, to Gornowicz et al., describes a novel thermoplastic silicone elastomer comprising mixing a thermoplastic resin such as a polyolefin or poly(butylene terephthalate) with a diorganic polysiloxane having at least two alkenyl groups and organo hydrido silicone compounds and subsequently dynamically vulcanizing the system via a hydrosilation cure using a platinum catalyst. Only polyolefin or poly(butylene terephthalate) resins were disclosed as being suitable because many other resins contain certain groups which can “poison” the platinum catalyst. In addition, other resins were unsuitable because they contain residual unsaturation, which depletes the Si—H availability for the crosslinking reaction.
The '715 patent discloses a thermoplastic elastomer product which is a blend of thermoplastic and polyorganosiloxane resin. The reaction process involves a dibutyltindilaurate catalyzed condensation reaction of a polydiorganosilanol with a hydrodiodiorgano silane. This reference is devoid of any teaching with respect to a curable resin composition containing discrete thermoplastic particles in a pseudo or fully interpenetrating network.
U.S. Pat. Nos. 5,834,537 and 6,225,373, both to Gotro et al., disclose a curable thermosetting resin composition with enhanced fracture toughness as a result of incorporating reactive thermoplastic oligomers therein is described. The curable composition of the '537 patent describes a thermoset resin containing fluorine and at least one fluorine-containing thermoplastic, soluble in the thermoset, which is cured between 100 to 325° C., to yield a toughened phase-separated thermoplastic in a thermoset. The '373 patent discloses a similar composition, wherein the thermoset contains bromine and optionally other halogens.
U.S. Pat. No. 5,709,948 to Perez et al. describe a curable composition made by mixing an epoxy resin and curative with prepolymerized hydrocarbon resin and a fully prepolymerized functionalized polyolefin resin and curing said composition in a twin screw heated extruder following by extrusion. The final composition is a thermoplastic polyolefin in an epoxy resin, the polyolefin being chemically linked to the epoxy via the functionalized groups. The composition described above is a thermoplastic-modified epoxy resin.
U.S. Pat. No. 4,954,195 to Turpin describes a process to produce thermoset composites by dissolving thermoplastic particles of polyimide, polyphenylenesulfide, or polyethersulfone, ranging in size from about 10 to 80 microns, in a thermoset resin at the cure temperature of the thermoset, which can be a bismaleimide (BMI) or an epoxy. This composition is applied as fiber reinforcements to form prepregs, and cured at elevated temperatures to form a soluble thermoplastic in a rigid thermoset composite.
U.S. Pat. No. 5,952,416 to Tani et al. describe a thermosetting resin composition containing thermoplastic polyolefin powder of particle sizes up to 200 μm and potassium titanate filler fibers, said thermoset being derived from unsaturated polyesters, vinyl esters, phenolic resin, polyester/styrene or epoxy phenolics. Such a product is said to provide high slidability and abrasion resistance.
Other patents disclose the use of pseudo or semi-IPN's. For example, U.S. Pat. No. 4,714,739 discloses pseudo or semi-interpenetrating polymer (IPN) networks of silicones in thermoplastic matrices. The pseudo or semi-IPN's are formed by vulcanization of vinyl containing silicones or hybrid vinyl containing polymers formed by reaction of a hydrid-containing silicone with a vinyl thermoplastic polymer or unsaturated group-containing polymer such as poly(1,2-butadiene), with a hydride-containing silicone polymer using a platinum catalyst, in the presence of a thermoplastic resin such as nylon or polyethylene or polyester urethane at the melt temperature of the thermoplastic resin. The level of semi-IPN in the thermoplastic matrix is demonstrated at low levels of 5 to 20%. An intimate blend of the mixture is cured and melt-processed in the injection molding equipment at 350° C. The process requires intimate mixing of the melted thermoplastic with reacting silicone components at the melt temperature of the thermoplastic to form the semi-IPN thermoplastic elastomer. This patent discloses the production of a semi-IPN, containing about 5 to 20% silicone networks, in a thermoplastic matrix. No disclosure is provided with respect to curable compositions which can be reshaped or reformed subsequent to cure.
U.S. Pat. No. 5,866,258 to Lucas describes an IPN composition made in two steps: (1) crosslinking an aqueous dispersion of isocyanate based thermoplastic polymer and unsaturated monomers containing latent reactive functional groups using peroxide initiators to form a stable coating composition, and (2) further reacting the composition at elevated temperatures (95-175° C.) to form a self-crosslinked IPN composition. Vinyl monomers containing latent reactive groups include acrylic acid, allylalcohol, vinyl acryloxy ether, 2-isocyanatoethylmethacrylate, meta-isopropenyl-α, α-dimethyl benzylisocyanate, and hydroxyethylacrylate. This IPN composition does not result in a discrete, meltable thermoplastic component in a cured elastomeric matrix.
U.S. Pat. No. 5,980,923 to Dillon describes an elastomeric membrane or sheeting material formed from polydimethylsilicone (PDMS) and polytetrafluoroethylene (PTFE). A semi-interpenetrating network is formed by causing a matrix of PDMS and a matrix of PTFE to be formed in situ. The membrane, which is used for treatment of dermatological scars, is further coated with a separate PDMS layers to form the final product.
At the present, there is a need for curable compositions which can be reshaped or reformed subsequent to cure, without destroying or degrading the crosslinked structure.